Field of the Invention
The invention concerns techniques for selectively carrying out a magnetic resonance measurement sequence with specific machine control parameters of a magnetic resonance system, as a function of a comparison of the machine control parameters with reference control parameters that are stored in a database.
Description of the Prior Art
An MR measurement sequence is typically carried out as part of magnetic resonance (MR) imaging. The MR measurement sequence includes—as well as radio frequency pulses and read-out windows, for example—the switching (activation) of gradient pulses of a gradient system, i.e. the time-dependent application of gradient fields by feeding power to gradient coils of a gradient system. The switching of the gradient pulses typically enables local encoding of the MR data acquired as part of MR imaging to be achieved. With typically dimensioned gradient systems it can be necessary for currents of up to 900 Amperes to flow through the gradient coils.
Such currents, or comparably high currents that flow through the gradient coils—especially in conjunction with gradient pulses switched rapidly over time—can give rise to significant technical problems. It can thus be necessary during an MR measurement sequence to switch the gradient pulses within a few milliseconds. The rapid switching of the gradient pulse results in a correspondingly rapid change in the gradient fields employed. The strong and rapid change of these magnetic fields over time typically result in significant mechanical force influences in the gradient system. This frequently results in vibrations and mechanical distortions of the gradient coils, thus generally in mechanical force paths, which can be transmitted to surrounding components of the magnetic resonance system. As a result of such mechanical force paths, a large amount of noise can develop in and around the magnetic resonance system. The result is noise stress for the patient, so that countermeasures can be necessary, otherwise the comfort of the patient is reduced. An (acoustic) frequency spectrum of such mechanical movements corresponds in such cases to a Fourier transformation of a timing sequence of the gradient pulse during execution of the magnetic resonance measurement sequence. As a result of resonance effects of the gradient system or the magnetic resonance scanner, it can occur that the switching of a timing sequence of gradient pulses with specific frequency portions in so-called forbidden frequency bands has especially great effects, i.e. causes an increased flow of mechanical force in the gradient system. Thus, in such a case, the result can be an especially large amount of noise developing, strong vibrations, or an increased amount of heat developing. If the basic magnetic field is generated by superconducting coils in a cryostat, the result of a large amount of heat developing can be evaporation of coolant, e.g. helium, for the cryostat. Therefore efforts are being made, when carrying out the MR measurement sequence, to avoid a timing sequence of the gradient pulses that results in such an increased mechanical force flow in the gradient system.
There are various known solutions to this problem. For example, before carrying out the MR measurement sequence, it is possible to analyze and evaluate the timing sequence of the gradient pulses and in this way determine, or predict computationally, which frequencies are likely to be excited. In order to avoid resonance effects or increased mechanical force flow in the gradient system, the measurement sequence developer is typically obliged to avoid specific forbidden frequency bands. This can be implemented, for example, by specific time spacings between spin echoes or gradient echoes not being allowed. Such known techniques have the disadvantage that the frequency spectrum that is excited by the timing sequence of the gradient pulses is able to be computed only to a limited extent, or with comparatively high computing outlay. This can restrict the practicality of such techniques, especially with limited resources as regards computing capacity and/or time.
Therefore, in a further known approach, the excited frequencies are monitored with a so-called frequency monitor while the MR measurement sequence is being carried out. Such frequency monitoring checks the excited frequencies for the different gradient axes. Frequency monitoring can be implemented for example by a real-time Fourier transformation of the timing sequence of the gradient pulses, especially for example the timing sequence of the current flows through the gradient coil. As part of the real-time frequency monitoring for example at least one forbidden frequency band Δω and an associated maximum allowed current strength Amax can be predetermined. If Amax is exceeded when the MR measurement sequence is carried out in the corresponding forbidden frequency band Δω the carrying out of the MR measurement sequence is interrupted. Such aborting of the carrying out of the MR measurement sequence can be disadvantageous for the performance of the MR system. Thus, MR data acquired before the sequence abortion can become unusable and it can be necessary subsequently to create a new MR measurement sequence. All this can be time-intensive and susceptible to errors. If, for example, the MR measurement sequence is not changed sufficiently, or the same MR measurement sequence is carried out again at a later point in time (for example by another user), then the same error can occur once again.